Hard mask structure for patterning of materials

ABSTRACT

Techniques for magnetic device fabrication are provided. In one aspect, a method of patterning at least one, e.g., nonvolatile, material comprises the following steps. A hard mask structure is formed on at least one surface of the material to be patterned. The hard mask structure is configured to have a base, proximate to the material, and a top opposite the base. The base has one or more lateral dimensions that are greater than one or more lateral dimensions of the top of the hard mask structure, such that at least one portion of the base extends out laterally a substantial distance beyond the top. The top of the hard mask structure is at a greater vertical distance from the material being etched than the base. The material is etched.

FIELD OF THE INVENTION

The present invention relates to patterning of materials and, moreparticularly, to improved hard mask structures for patterning ofmaterials.

BACKGROUND OF THE INVENTION

Devices, such as magnetic memory devices, may be formed using standardpatterning techniques. Magnetic memory devices use magnetic memory cellsto store information. Information is stored in such magnetic memorydevices as the orientation of the magnetization of a storage layer inthe magnetic memory cell as compared to the orientation of themagnetization of a reference layer in the memory cell. The magnetizationof the storage layer may be oriented parallel or anti-parallel to thereference layer, representing either a logic “0” or a “1.” One type ofmemory cell, a magnetic tunnel junction (MTJ), comprises a storage layerand a reference layer separated by a tunnel barrier.

Patterning of the magnetic memory cells can be done with reactive ionetching, e.g., in a manner similar to that used to pattern transistorsin complementary metal oxide semiconductor (CMOS) technology.Alternatively, patterning can be done with ion beam etching, e.g., in amanner similar to that used to pattern read heads for magnetic diskdrives. During etching to pattern the magnetic memory cells, however,nonvolatile materials that have been removed from the wafer surfaces maybecome re-deposited on portions of the cell, having deleterious effects.The re-deposited materials can result in a poorly-defined cell shape,e.g., by terminating the edges with an ill-defined material. Further, inthe case of an MTJ, for example, the re-deposited material can causeshorting across the tunnel barrier. Thus, use of conventional etchingtechniques can negatively affect the properties of the cell. Thisproblem may be further worsened by the use of certain materials that areparticularly difficult to etch, as they do not easily form volatilecompounds when interacting with etch gasses at normal processingtemperatures.

Therefore, etching techniques that minimize or eliminate the effects ofre-deposited materials would be desirable.

SUMMARY OF THE INVENTION

Techniques for patterning of materials using a hard mask structure areprovided, in accordance with an illustrative embodiment of the presentinvention. In one aspect of the invention, a method of patterning atleast one material comprises the following steps. A hard mask structureis formed on at least one surface of the material to be patterned. Thehard mask structure is configured to have a base, proximate to thematerial, and a top opposite the base. The base has one or more lateraldimensions that are greater than one or more lateral dimensions of thetop of the hard mask structure, such that at least one portion of thebase extends out laterally a substantial distance beyond the top. Thetop of the hard mask structure is at a greater vertical distance fromthe material being etched than the base. For example, each at least oneportion of the base extending out laterally beyond the top, when viewedin lateral cross section, has a longest lateral dimension that isbetween about 20 percent and about 40 percent, e.g., between about 30percent and about 40 percent, of the longest lateral dimension of thebase. The material is etched.

A more complete understanding of the present invention, as well asfurther features and advantages of the present invention, will beobtained by reference to the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional image of a magnetic device having beenformed using conventional etching techniques;

FIG. 2 is an image of a portion of a magnetic device being formed usingconventional etching techniques;

FIG. 3 is a lateral cross-sectional image of a magnetic device beingformed using the present techniques according to an embodiment of thepresent invention;

FIG. 4 is a diagram illustrating an exemplary methodology for forming amagnetic device according to an embodiment of the present invention;

FIG. 5 is a diagram illustrating another exemplary methodology forforming a magnetic device according to an embodiment of the presentinvention;

FIGS. 6A-B are diagrams illustrating yet another exemplary methodologyfor forming a magnetic device according to an embodiment of the presentinvention;

FIGS. 7A-D are images illustrating hard mask structures graded using thepresent techniques according to an embodiment of the present invention;and

FIGS. 8A-D are images of hard mask structures graded using the presenttechniques after etching of underlying nonvolatile materials accordingto an embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Prior to describing the inventive aspects of the present invention,FIGS. 1 and 2, directed to conventional etching techniques, will bedescribed. It is to be understood that the various layers and/or regionsshown in the accompanying figures may not be drawn to scale.Furthermore, one or more semiconductor layers of a type commonly used insuch integrated circuit structures may not be explicitly shown in agiven figure for ease of explanation. This does not imply that thesemiconductor layers not explicitly shown are omitted in the actualintegrated circuit device.

FIG. 1 is a cross-sectional image of magnetic device 102 having beenformed using conventional etching techniques. Portion 104 of magneticdevice 102 is shown amplified in FIG. 2, described below. Magneticdevice 102 comprises a magnetic tunnel junction (MTJ).

FIG. 2 is an image of portion 104 of magnetic device 102 (of FIG. 1). Asshown in FIG. 2, magnetic device 102 comprises storage, e.g., soft,layer 202, which comprises magnetic layers 203 and 213 separated bycoupling layer 204. Magnetic device 102 also comprises reference layer210 separated from storage layer 202 by tunnel barrier 212. Cap/hardmask/etch stop layers 206, 207, 208, and 209 are present on top ofstorage layer 202.

During etching of magnetic device 102, material dislodged by the etchingmay become re-distributed, as indicated, e.g., by arrows 214. There-distributed material can undesirably re-deposit on portions ofmagnetic device 102, which is typically the case during etching ofnonvolatile materials.

Specifically, the re-distributed materials may re-deposit on portions ofmagnetic device 102 that have already been etched. For example, there-distributed material from etching of reference layer 210 may bere-deposited on portions of storage layer 202 and tunnel barrier 212,which can unfavorably result in magnetic device 102 having a variableshape and/or size, e.g., as indicated by arrow 216. This re-depositionof materials on the, sidewalls of the device during etching is referredto herein as “sidewall re-deposition.”

The re-distributed materials can also become re-deposited so as to forma continuous electrical contact from storage layer 202 to referencelayer 210, shorting out tunnel barrier 212. As such, the magnetic andelectronic behavior of the device formed may be adversely affected.

While the present description provides, as an exemplary model, for theformation of magnetic devices, such as magnetic random access memory(MRAM) devices, it is to be understood that the present techniques arebroadly applicable to the patterning of materials, including nonvolatilematerials not readily etched by standard reactive ion etching (RIE)techniques. Other suitable applications for the present techniquesinclude, but are not limited to, etching of capacitors made with, e.g.,nonvolatile platinum (Pt) or iridium (Ir)-based electrodes, etching offerroelectric materials (such as lead zirconium titanate (PLZT)), forferroelectric random access memory (FRAM), etching of phase changematerials, e.g., chalcogenides for phase change memory (PCM), andetching of metal gate materials for transistor applications such ascomplementary metal oxide semiconductor (CMOS), with materialsincluding, but not limited to, molybdenum (Mo), Ir, rhodium (Rh),rhenium (Re), ruthenium (Ru), nickel (Ni), tungsten (W) and tantalum(Ta).

FIG. 3 is a lateral cross-sectional image of magnetic device 302 beingformed using the present techniques. According to one exemplaryembodiment, magnetic device 302 comprises an MTJ. Magnetic device 302comprises storage, e.g., soft, layer 304, which itself comprisesmagnetic layers 306 and 310 in an anti-parallel configuration, separatedby coupling layer 308. Magnetic device 302 also comprises referencelayer 312 separated from storage layer 304 by barrier layer 314.Reference layer 312 may comprise a single layer, or alternatively,multiple layers.

During formation of magnetic device 302, hard mask structure 315,comprising thin hard mask layer 316 and thick hard mask layer 318, isemployed to prevent, at least in part, the undesirable effects ofre-deposited nonvolatile material which becomes re-distributed duringetching, e.g., as indicated by arrow 320. The term “nonvolatilematerial,” as used herein, indicates that the material does not readilyform volatile compounds through reaction with the etch gasses, as wouldbe the case for conventional RIE at temperatures less than about 200° C.Such nonvolatile materials will adhere to the exposed surfaces of thewafer and the etch chamber, rather than being carried away as gas-phaseby-products of the etch. The term “hard mask,” as used herein, generallyrefers to any non-photoresist material.

Specifically, hard mask structure 315 is configured to have a baseproximate to the material being etched and a top opposite the base. Thebase has one or more lateral dimensions that are greater than one ormore lateral dimensions of a top of hard mask structure 315, such thatat least one portion of the base extends out laterally a substantialdistance beyond the top. According to one exemplary embodiment, whenviewed in lateral cross-section, each portion of the base extending outlaterally beyond the top has a longest lateral dimension that is betweenabout 20 percent and about 40 percent of the longest lateral dimensionof the base. For example, each portion of the base extending outlaterally beyond the top has a longest lateral dimension that is betweenabout 30 percent and about 40 percent of the longest lateral dimensionof the base.

The top of hard mask structure 315 is at a greater vertical distancefrom the material being etched than the base. For example, hard maskstructure 315 may comprise a base, e.g., comprising thin hard mask layer316, and a top, e.g., comprising thick hard mask layer 318, so as toform a “top hat” configuration.

Further, hard mask structure 315 may be a circular, or substantiallycircular (e.g., elliptical) structure, when viewed as a lateralcross-section, wherein the base has a diameter that is greater than adiameter of the top of hard mask structure 315, distal to the materialbeing etched. See, for example, FIGS. 7A-D which show top-down andtilted-angle side views of elliptical structures similar to that seen incross-section in FIG. 3. Specifically, according to one exemplaryembodiment, hard mask structure 315 is circular, and has a diameter 324at its base that is greater than diameter 326 at its top.

However, a circular, or substantially circular hard mask structure ismerely an exemplary configuration. Other suitable hard mask structureconfigurations, for example, include, but are not limited to,rectangular or linear configurations. Further, each portion of the baseextending laterally beyond the top has a first slope and at least onesidewall of the top has a second slope, the first slope being differentfrom the second slope. Specifically, taking the interface of the baseand the top as an origin, the slope of the top, which may include aslope of infinity (or, equivalently 90 degrees), is substantiallygreater than the slope of the base, which may include a slope of zeroor, in some exemplary embodiments, a negative slope, such that anoutermost laterally extending portion of the base is thicker than aportion of the base at the origin.

Further, as shown in FIG. 3, the progression of the diameter beinglarger at the base to smaller at the top may be in a step-like fashion(e.g., an abrupt change in diameter from thin hard mask layer 316 tothick hard mask layer 318). However, as is shown in FIG. 5, anddescribed below, the hard mask structure may have a taperedconfiguration.

As a result, hard mask structure 315 is thickest at its center, e.g.,center thickness 322. As will be described in detail below, thisconfiguration is desirable for its utility in connecting magnetic device302 to associated wiring levels above the device.

Having a hard mask structure with a diameter at its base that is greaterthan a diameter at its top has several notable advantages. For example,with such a hard mask structure, the shadowing of incoming etchant isminimized, which increases the sputter yield of the sidewallre-deposition. As such, the sidewalls are kept clean during etching ofunmasked regions. Also, the present hard mask configuration minimizesthe surface area of the hard mask that can act as a collector ofnonvolatile residues just above the sensitive exposed edges of the softlayer and tunnel barrier. Therefore, less material will be present to besputtered downward, and e.g., shunt the tunnel barrier. Referring toFIG. 3, sidewall re-deposits on thick hard mask 318 may again (duringthe same etch process that created the re-deposited material) bedisplaced downwards (forward-sputtered) to land on top of thin hard mask316. According to the present techniques, this material is, however,kept at a substantial distance from the sensitive edges of the activelayers, e.g., storage layer 304, and barrier layer 314. The reduced hardmask thickness (e.g., from layer 316 only) near the sensitive edgeresults in less available material to be forward-sputtered, and thusless risk of shunting the device, e.g., shorting barrier layer 314, oraltering its behavior.

As mentioned above, according to one exemplary embodiment, the presenthard mask structure is used in forming an MTJ wherein portions of thehard mask structure remain, post-etching, as an integral part of theMTJ. According to this exemplary embodiment, the hard mask structure iselectrically conductive. As such, the present hard mask structure isadvantageous as it is thickest at its center and provides for an easyconnection to wiring levels associated with the device. For example, inFIG. 3, hard mask structure 315 is thickest at its center, e.g.,thickness 322, which provides for a self-aligned connection to wiringlayer 330 above the device, see below description.

The use of a hard mask structure comprising a thick/thin hard maskbilayer during the formation of a magnetic tunnel junction is shownschematically, for example, in FIG. 4, and described below.

FIG. 4 is a diagram illustrating an exemplary methodology for forming amagnetic device. According to one exemplary embodiment, the magneticdevice formed comprises an MTJ. In step 402, the layers used to form themagnetic device are provided. Specifically, magnetic layer 408 willserve as a reference layer for the magnetic device. Suitable materialsfor forming magnetic layer 408 include, but are not limited to, one ormore of iridium-manganese (IrMn), platinum-manganese (PtMn), cobalt-iron(CoFe), Ru, cobalt-iron-boron (CoFeB), nickel-iron (NiFe),chromium-molybdenum (CrMo), Ta, and tantalum nitride (TaN). Magneticlayer 408 may comprise a single layer, or alternatively, multiplelayers.

Barrier layer 410, deposited on a top side of magnetic layer 408, willserve as a tunnel barrier for the magnetic device. Suitable materialsfor forming barrier layer 410 include, but are not limited to one ormore of aluminum oxide (Al₂O₃), magnesium oxide (MgO), boron nitride(BN), silicon oxide (e.g., SiO₂), and nonstoichiometric variantsthereof.

Magnetic layer 412, deposited on a side of barrier layer 410 oppositemagnetic layer 408, will serve as a bottom storage layer, e.g., of amulti-layer storage layer configuration, for the magnetic device.Suitable materials for forming magnetic layer 412 include, but are notlimited to one or more of NiFe, CoFe, CoFeB, nickel-cobalt-iron (NiCoFe)and multi-layers comprising one or more of the foregoing. Further,magnetic layer 412 may have a thickness of up to about ten nm, forexample, from about two nm to about ten nm. According to an exemplaryembodiment, magnetic layer 412 comprises NiFe and is about five nmthick.

Coupling layer 414, deposited on a side of magnetic layer 412 oppositebarrier layer 410, will serve as a coupling layer for magnetic layer 412and magnetic layer 416 (described below), e.g., for a multi-layerstorage layer configuration. Suitable materials for forming couplinglayer 414 include, but are not limited to, one or more of Ru, CrMo andTaN. Further, coupling layer 414 may have a thickness of up to about tennm, for example, from about 0.5 nm to about ten nm. According to anexemplary embodiment, coupling layer 414 comprises Ru and is about fivenm thick.

Magnetic layer 416, deposited on a side of coupling layer 414 oppositemagnetic layer 412, will serve as a top storage layer, e.g., of amulti-layer storage layer configuration, for the magnetic device.Suitable materials for forming magnetic layer 416 include, but are notlimited to, one or more of NiFe, CoFe, CoFeB, NiCoFe and multi-layerscomprising one or more of the foregoing. Further, magnetic layer 416 mayhave a thickness of up to about ten nm, for example, from about two nmto about ten nm. According to an exemplary embodiment, magnetic layer416 comprises NiFe and is about five nm thick.

Cap/etch stop layer 418 is deposited on a side of magnetic layer 416opposite coupling layer 414. This layer is optional. For example,depending on the etch chemistries and/or materials used, magnetic layer416 may act as an etch stop layer such that cap/etch stop layer 418 isnot required. Further, separate cap and etch stop layers may beemployed. For example, a cap layer may be needed for stability of thematerials below it, but may not act as an etch stop layer. Therefore, aseparate material may be employed, above the cap layer, as an etch stop.By way of example only, a cap/etch stop layer comprising Ru may allowoxygen to diffuse through, and alter the properties of, the underlyingmagnetic layers. In this instance, it may be desirable to additionallyemploy a layer comprising, e.g., TaN, below the Ru layer. According tothis exemplary configuration, the layer comprising TaN would act as acap layer, and the layer comprising Ru would act as an etch stop layer.

Suitable materials for forming cap/etch stop layer 418 include, but arenot limited to Ru, Ta and TaN. Further, cap/etch stop layer 418 may havea thickness of up to about ten nm, for example, from about two nm toabout ten nm. According to an exemplary embodiment, cap/etch stop layer418 comprises Ru and is about ten nm thick.

Thin hard mask layer 420 is deposited on a side of cap/etch stop layer418 opposite magnetic layer 416. Suitable materials for forming thinhard mask layer 420 include, but are not limited to, one or more of TaN,W, Ta, aluminum (Al) and Ru. Further, thin hard mask layer 420 may havea thickness of up to about 40 nm, for example, from about five nm toabout 40 nm. According to an exemplary embodiment, thin hard mask layer420 comprises TaN and is about 20 nm thick.

Thick hard mask layer 422 is deposited on a side of thin hard mask layer420 opposite cap/etch stop layer 418. As will be described in steps 404and 406, below, thin hard mask layer 420 and thick hard mask layer 422comprise hard mask bilayer structure 423 and will be employed duringformation of the magnetic tunnel junction. Suitable materials forforming thick hard mask layer 422 include, but are not limited to, oneor more of titanium nitride (TiN), W, Al, Ta, TaN and silicon (Si).Further, thick hard mask layer 422 may have a thickness of up to about40 nm, for example, from about five nm to about 40 nm. According to anexemplary embodiment, thick hard mask layer 422 comprises TiN and isabout 20 nm thick.

As shown in step 402, hard mask bilayer structure 423, e.g., thin hardmask layer 420 and thick hard mask layer 422, are etched, e.g., using adry etch process. The etching of thin hard mask layer 420 and thick hardmask layer 422 stops at the layer immediately beneath thin hard masklayer 420, e.g., cap/etch stop layer 418 in this embodiment. After this“hard mask open” step, hard mask bilayer structure 423 has a footprintthat is approximately the desired size of the magnetic device. See, forexample, step 406, described below.

In step 404, a gentle wet etch process is used to taper thick hard masklayer 422. Suitable wet etch processes include, but are not limited to,those comprising a hydrogen peroxide/ammonium hydroxide (H₂O₂/NH₄OH) wetetchant, e.g., for thick hard mask layer 422 comprising TiN, and anammonium hydroxide (NH₄OH) wet etchant, e.g., for thick hard mask layer422 comprising Al. During the tapering of thick hard mask layer 422, thethickness of thick hard mask layer 422 may decrease, e.g., by up toabout 50 nm, as a result of the etching. For example, if thick hard masklayer 422 was originally about 150 nm thick, after wet etching it may beabout 100 nm thick (at its thickest point). Unless there is residualmasking of the top of thick hard mask layer 422, the thickness willdecrease by approximately the same amount that the sidewalls arelaterally recessed, as a consequence of the isotropy of the wet etchingbeing used.

The wet etch process employed for tapering thick hard mask layer 422should be selective for etching primarily thick hard mask layer 422. Forexample, in one embodiment, an H₂O₂/NH₄OH wet etchant is employed toselectively etch thick hard mask layer 422 (comprising TiN) to theexclusion of thin hard mask layer 420 (comprising TaN) and cap/etch stoplayer 418 (comprising Ru), which are substantially immune to etching byan H₂O₂/NH₄OH wet etchant.

As shown in step 404 of FIG. 4, selectively etching thick hard masklayer 422 leaves behind a well-defined mask edge, determined by thejuncture between thin hard mask layer 420 and cap/etch stop layer 418.The lateral dimensions of this juncture, e.g., as indicated by arrow424, indicates approximately the desired final lateral dimensions of themagnetic device.

As described, for example, in conjunction with the description FIG. 3above, hard mask bilayer structure 423 is configured to have a base,proximate to the material being etched, and a top opposite the base. Thebase has one or more lateral dimensions that are greater than one ormore lateral dimensions of a top of hard mask bilayer structure 423,such that at least one portion of the base extends out laterally asubstantial distance beyond the top. According to one exemplaryembodiment, when viewed in lateral cross section, each portion of thebase extending out laterally beyond the top has a longest lateraldimension that is between about 20 percent and about 40 percent of thelongest lateral dimension of the base. For example, each portion of thebase extending out laterally beyond the top may have a longest lateraldimension that is between about 30 percent and about 40 percent of thelongest lateral dimension of the base.

The top of hard mask bilayer structure 423 is at a greater verticaldistance from the material being etched than the base. For example, hardmask bilayer structure 423 may comprise a top, e.g., comprising thickhard mask layer 422, and a base, e.g., comprising thin hard mask layer420. Hard mask bilayer structure 423 may be a circular, or substantiallycircular (e.g., elliptical) structure, wherein the base has a diameterthat is greater than a diameter of the top of hard mask bilayerstructure 423, distal to the material being etched. See, for example,FIGS. 7A-D. Specifically, according to one exemplary embodiment, hardmask bilayer structure 423 is circular, and has a diameter 424 at itsbase that is greater than diameter 426 at its top.

Further, each portion of the base extending out laterally beyond the tophas a first slope associated with its upper surface, and at least onesidewall of the top has a second slope, the first slope being differentfrom the second slope. Specifically, the slope of the sidewall of thetop is greater than the slope of the upper surface of the base.According to the teachings presented herein, each of the first slope andthe second slope, in any of the embodiments disclosed herein, may have avalue of between zero and infinity. For example, as shown in step 404 ofFIG. 4, the slope of the base is zero and the slope of the top is about80 degrees near its uppermost surface.

In step 406, a dry etch process is used to etch cap/etch stop layer 418,magnetic layer 416, coupling layer 414, magnetic layer 412 and into orthrough barrier layer 410. Any suitable dry etch process that iscompatible with using thin hard mask layer 420 for shape definition ofthe magnetic device may be employed. For example, when using a Ta orTi-based thin hard mask layer 420, a highly-selective oxygen-basedchemistry, such as CO—NH₃, may be employed, either alone or incombination with another etching technique, such as ion beam etching(IBE) (pure sputtering without a reactive component) or wet chemicaletching. According to one exemplary embodiment, an oxygen-based etch isperformed to etch through cap/etch stop layer 418 followed by IBE forthe remainder.

Alternatively, IBE may be used exclusively throughout. If so, it may bedesirable to increase the thickness of thin hard mask layer 420, e.g.,to be up to about ten nm thicker than the total thickness of the layersbeing etched, so that it is not fully eroded during etching.

The above techniques will minimize, or prevent, re-distributednonvolatile materials from being re-deposited on, and negativelyaffecting performance of, the magnetic layers in the device. Forexample, the above techniques reduce shadowing of incoming ions at theetch front, as defined by the regions not masked by thin hard mask layer420, e.g., during IBE or RIE, because thick hard mask layer 422 isrecessed away from the edge of the etch front. As a result, an increasedsputter yield of material (e.g., material is sputtered-away), ascompared to etching with conventional masking techniques, is experiencedat regions of the device wherein re-deposition is most detrimental. Anynonvolatile material that is not sputtered away entirely from the wafersurface will be gettered near thick hard mask layer 422, e.g., and awayfrom the sensitive etch front that exposes the edges of magnetic layers408, 412, 414 and 416, and barrier layer 410. As such, the nonvolatileetch by-product (re-distributed material) will be kept away from theedges of magnetic layers 408, 412, 414 and 416, and barrier layer 410.Further, forward-sputtering of re-deposited materials back into the etchfront is significantly reduced when compared with conventional etchingtechniques, because this forward-sputtering takes place at a distancefrom the etch front. Specifically, any material forward-sputtered fromthe edges of thick hard mask layer 422 will be deposited on the portionsof thin hard mask layer 420 extending out beneath thick hard mask layer422. The sensitive edges of layers 408, 410, 412, 414, and 416 cantherefore be kept free of re-deposited material through the reducedforward-sputtering of etch by-products back into the sensitive region,and the increased sputter yield of material near the sensitive edges,because of reduced shadowing of incident etchant atoms, ions, ormolecules.

The techniques described herein, e.g., using an isotropic wet etch togenerate the desired hard mask profile, after a well-controlled verticaldry etch is used to define the final shape of the device, arescaling-friendly, i.e., can be used to create devices of variousdimensions, including scaled-down versions of the devices presentedherein. For example, according to the present techniques, geometrictaper angle does not have to limit the scaling/minimum devicedimensions, because the taper (slope) is shallow only over a smallfraction of the total hard mask width, and a near-vertical sidewall isused to build up the bulk of the hard mask thickness for integrationwith associated wiring layers of the device.

Further, as mentioned above, the hard mask structure might comprise anelectrically conductive material and can be used to form a contact towiring levels. According to this embodiment, an isotropic wet etch maybe used to pattern the hard mask structure. An isotropic wet etch servesto etch all faces of the hard mask structure at approximately the samerate. Therefore, a conductive hard mask structure, patterned accordingto the present techniques, using an isotropic wet etch, will comprise anelectric contact centered, or substantially centered, on the device.Such a contact is referred to herein as a “self-aligned” contact, as itis formed during formation of the device. This embodiment allows fordense packing of devices, e.g., because it eliminates the complexityassociated with having additional via levels for connection. Theself-aligned conductive hard mask provides a straightforward way toconnect wiring atop the device. See, for example, in FIG. 3, describedabove, wherein wiring layer 330 contacts conductive thick hard masklayer 318 without risking wiring layer 330 inadvertently contactinglayers beneath the tunnel barrier and shunting the device. Thick hardmask layers, such as thick hard mask layers 318 and 422, are generallyeasier to integrate with ensuing wiring levels because there is less ofa danger of these wiring layers contacting the layers beneath barrierlayer 314. There are, however, limits on the thickness of the hard masklayers. For example, if the thick hard mask layer negatively affects theetch through the aforementioned shadowing and forward-sputteringprocesses, and thus results in loss of performance, it may be too thick.For example, in MTJ devices wherein wiring layers similar to wiringlayer 330 are used to generate a magnetic field to switch the device,the power required to switch the device will increase as wiring layer330 is moved away from the active magnetic layers. At the same time,while a thin hard mask structure (e.g., hard mask bilayer structure 423with thin, or nonexistent thick hard mask layer 422) can solve problemswith etch re-deposition, it can be difficult to integrate with ensuingwiring layers because it does not provide a means for self-alignedcontacting. The hard mask structures presented herein merge the benefitsof a thick hard mask layer with the benefits of a thin hard mask layerto provide an exemplary means of contacting ensuing wire layers, whileat the same time providing a means of defining structures during etch.

FIG. 5 is a diagram illustrating another exemplary methodology forforming a magnetic device. The formation of the magnetic device shown inFIG. 5 comprises etching of nonvolatile materials. According to oneexemplary embodiment, the magnetic device formed comprises an MTJ. Instep 502, the layers used to form the magnetic device are provided.Specifically, magnetic layer 506 will serve as a reference layer for themagnetic device. Suitable materials for forming magnetic layer 506include, but are not limited to, those described in conjunction with thedescription of magnetic layer 408 above. Magnetic layer 506 may comprisea single layer, or alternatively, multiple layers.

Barrier layer 508, deposited on a top side of magnetic layer 506, willserve as a tunnel barrier for the magnetic device. Suitable materialsfor forming barrier layer 508 include, but are not limited to, thosedescribed in conjunction with the description of barrier layer 410above.

Magnetic layer 510, deposited on a side of barrier layer 508 oppositemagnetic layer 506, will serve as a bottom storage layer, e.g., of amulti-storage layer configuration, for the magnetic device. Suitablematerials for forming magnetic layer 510 include, but are not limitedto, those described in conjunction with the description of magneticlayer 412 above.

Coupling layer 512, deposited on a side of magnetic layer 510 oppositebarrier layer 508, will serve as a coupling layer for magnetic layer 510and magnetic layer 514 (described below), e.g., for a multi-layerstorage layer configuration. Suitable materials for forming couplinglayer 512 include, but are not limited to, those described inconjunction with the description of coupling layer 414 above.

Magnetic layer 514, deposited on a side of coupling layer 512 oppositemagnetic layer 510, will serve as a top storage layer, e.g., of amulti-layer storage layer configuration, for the magnetic device.Suitable materials for forming magnetic layer 514 include, but are notlimited to, those described in conjunction with the description ofmagnetic layer 416 above.

Cap/etch stop layer 516 is deposited on a side of magnetic layer 514opposite coupling layer 512. Suitable materials for forming cap/etchstop layer 516 include, but are not limited to, those described inconjunction with the description of cap/etch stop layer 418 above.Further, cap/etch stop layer 516 may have a thickness of up to about 20nm, for example, from about two nm to about 20 nm. According to anexemplary embodiment, cap/etch stop layer 516 comprises Ru and is aboutten nm thick.

Hard mask layer 518 is deposited on a side of cap/etch stop layer 516opposite magnetic layer 514. As will be described in step 504, below,hard mask layer 518 will be employed, e.g., as a hard mask structure,during formation of the magnetic device. Suitable materials for forminghard mask layer 518 include, but are not limited to, those described inconjunction with the description of thick hard mask layer 422 above.Further, hard mask layer 518 may have a thickness of up to about 200 nm,for example, from about 70 nm to about 200 nm. According to an exemplaryembodiment, hard mask layer 518 comprises TiN and is about 200 nm thick.Therefore, according to the exemplary embodiment shown in FIG. 5, thehard mask structure comprises a single layer, hard mask layer 518.

In step 504, etching is used to taper thick hard mask layer 518.According to one exemplary embodiment, wet etching techniques areemployed to etch hard mask layer 518.

As described, for example, in conjunction with the description of FIGS.3 and 4 above, the hard mask structure is configured to have a base,proximate to the material being etched, that has one or more lateraldimensions that are greater than one or more lateral dimensions of a topof the hard mask structure, such that at least one portion of the baseextends out laterally a substantial distance beyond the top. Accordingto one exemplary embodiment, when viewed in lateral cross section, eachportion of the base extending out laterally beyond the top has a longestlateral dimension that is between about 20 percent and about 40 percentof the longest lateral dimension of the base. For example, each portionof the base extending out laterally beyond the top may have a longestlateral dimension that is between about 30 percent and about 40 percentof the longest lateral dimension of the base.

The top of the hard mask structure is at a greater vertical distancefrom the material being etched than the base. For example, according toone exemplary embodiment, the base is defined as a portion of the hardmask structure extending up to about ten nm horizontally inward from thematerial being etched. The top is thus any portion of the hard maskstructure extending beyond up to about ten nm horizontally inward fromthe material being etched. By way of example only, in step 504 of FIG.5, the base and the top may be distinguished by line 524 which beginsabove the portion of the hard mask structure extending up to about tennm horizontally inward from the material being etched.

The hard mask structure may comprise a circular, or substantiallycircular (e.g., elliptical) structure, wherein the base has a diameterthat is greater than a diameter of the top of the hard mask structure,distal to the material being etched. See, for example, FIGS. 7A-D.Specifically, according to one exemplary embodiment, the hard maskstructure is circular, and has a diameter 520 at its base that isgreater than diameter 522 at its top. Further, each portion of the baseextending out laterally beyond the top has a first slope associated withits uppermost surface, and at least one sidewall of the top has a secondslope, the first slope being different from the second slope.Specifically, the slope of the top is greater than the slope of thebase.

Additionally, the hard mask structure may comprise other materials incombination with hard mask layer 518, including, but not limited to,dielectric masks, removable mandrels, and combinations comprising atleast one of the foregoing additional materials. When a dielectric maskis used, it may optionally be coated with a metallic spacer material.Coating the dielectric mask with a metallic spacer material provides forcontact with wiring levels above the hard mask structure, e.g., as inthe embodiment wherein the hard mask is conductive.

Removable mandrel techniques comprise the use of a disposable hard mask.Specifically, after etching of the device layers, the device and thehard mask structure are encapsulated in a dielectric. If not alreadyexposed, the hard mask is then exposed by an etchback or polish step.Next, the hard mask structure is selectively removed, e.g., like amandrel, and the newly formed cavity is filled with a conductivematerial that provides contact between the device and, e.g., a wiringlevel above the device. This approach provides for a self-alignedcontact even though a conductive hard mask is not used, and at the sametime utilizes the present mask structure to etch nonvolatile materials.

According to an exemplary embodiment, dry etch (instead of wet etch)chemistries are used to etch hard mask layer 518. It is desired that thedry etch employed has a high level of isotropy, e.g., produces an etchprofile that is substantially uniform in all directions resulting in aprofile wherein the hard mask structure is taller at its center than atits edges (and in the case of a self-aligned contact, as describedabove, the hard mask structure is substantially taller in the center ofthe device than at the device edges). Although typically a naturalresult of most wet etches, the achievement of isotropy with a dry etchrequires care to use low bias power in conjunction with a chemical etchcomponent. This is not particularly difficult for hard mask materialsthat can be etched using standard RIE (volatile byproduct) techniques.Chlorine (Cl), bromine (Br) or fluorine (F) can be used to etch hardmask materials like Ta, Ti, Al, W, Si and compounds created therefrom.

The use of isotropic etch techniques results in the desired hard maskshape, as described above, for the purpose of reducing shadowing of theetch front, and in aiding further process integration to contact theabove wiring layer. After the hard mask etch, the etch used to patternthe magnetic layers should be highly selective against etching the hardmask material, as the hard mask structure thins towards the edges of thedevice (see step 504). For example, a CO—NH₃ or methanol etch could beused in conjunction with a Ta or Ti-based hard mask structure to achievesuch selectivity. Although the embodiment presented in FIG. 5 is lesscomplicated that that of FIG. 4, the process as outlined in FIG. 4affords more control over device shape and size, and is thus preferablefor most applications wherein shape or size are important.

Further according to this exemplary embodiment, a layer comprising amaterial having an etch rate (in the hard mask etch process) that islower than hard mask layer 518, e.g., an intermediate hard mask layer(not shown), may be employed between hard mask layer 518 and cap/etchstop layer 516. Suitable materials for forming an intermediate hard masklayer include, but are not limited to Ta, TaN, Ti, TiN, Si, Al, W, Cr,Mo and combinations comprising at least one of the foregoing materials.Employing such a layer serves to increase the thickness of the hard masknear the edges, e.g., in a manner similar to that described in FIG. 4,but without an explicit step to recess hard mask layer 518 after theintermediate mask layer has been etched.

FIGS. 6A-B are diagrams illustrating yet another exemplary methodologyfor forming a magnetic device. According to one exemplary embodiment,the magnetic device formed comprises an MTJ. In step 602, the layersused to form the magnetic device are provided. Specifically, magneticlayer 610 will serve as a reference layer for the magnetic device.Suitable materials for forming magnetic layer 610 include, but are notlimited to, those described in conjunction with the description ofmagnetic layers 408 and 506 above. Magnetic layer 610 may comprise asingle layer, or alternatively, multiple layers.

Barrier layer 612, deposited on a top side of magnetic layer 610, willserve as a tunnel barrier for the magnetic device. Suitable materialsfor forming barrier layer 612 include, but are not limited to, thosedescribed in conjunction with the description of barrier layers 410 and508 above.

Magnetic layer 614, deposited on a side of barrier layer 612 oppositemagnetic layer 610, will serve as a bottom storage layer, e.g., of amulti-layer storage layer configuration, for the magnetic device.Suitable materials for forming magnetic layer 614 include, but are notlimited to, those described in conjunction with the description ofmagnetic layers 412 and 510 above.

Coupling layer 616, deposited on a side of magnetic layer 614 oppositebarrier layer 612, will serve as a coupling layer for magnetic layer 614and magnetic layer 618 (described below), e.g., for a multi-layerstorage layer configuration. Suitable materials for forming couplinglayer 616 include, but are not limited to, those described inconjunction with the description of coupling layers 414 and 512, above.

Magnetic layer 618, deposited on a side of coupling layer 616 oppositemagnetic layer 614, will serve as a top storage layer, e.g., of amulti-layer storage layer configuration, for the magnetic device.Suitable materials for forming magnetic layer 618 include, but are notlimited to, those described in conjunction with the description ofmagnetic layers 416 and 514, above.

A combined cap/etch stop/thin hard mask layer 620 is deposited on a sideof magnetic layer 618 opposite coupling layer 616. Suitable materialsfor forming cap/etch stop/thin hard mask layer 620 include, but are notlimited to Ru, Ta and TaN and bilayers comprising one or more of theforegoing. Further, cap/etch stop/thin hard mask layer 620 may have athickness of up to about 20 nm, for example, from about two to about 20nm. According to an exemplary embodiment, cap/etch stop/thin hard masklayer 620 comprises TaN and is about ten nm thick.

Hard mask layer 624 is deposited on a side of cap/etch stop/thin hardmask layer 620 opposite magnetic layer 618. Suitable materials forforming hard mask layer 624 include, but are not limited to, one or moreof TiN, W, Al, Ta, TaN and Si. Further, hard mask layer 624 may have athickness of up to about 200 nm, for example, from about 50 nm to about200 nm. According to an exemplary embodiment, hard mask layer 624comprises TiN and is about 100 nm thick.

As shown in step 602, hard mask layer 624 is etched. Specifically, hardmask layer 624 may be etched down to cap/etch stop/thin hard mask layer620, e.g., using a standard anisotropic RIE that creates near-verticalsidewalls.

In step 604, the edges of the etched hard mask layer 624 are coated withspacer material 626. The shape of spacer material 626 is obtained usingstandard techniques that include conformal film deposition andanisotropic etch that preferentially removes spacer material 626 fromall horizontal surfaces. Suitable spacer materials include, but are notlimited to, one or more of organic polymers, dielectrics and metals.According to the exemplary embodiment shown in step 604, spacer material626, at its base, has a width of between about ten nm and about 50 nm.

In step 606, the final hard mask structure is formed when cap/etchstop/thin hard mask layer 620 is etched and then spacer material 626 isremoved, e.g., with a dry or wet etch that is selective against etchinghard mask 624 and cap/etch stop/thin hard mask layer 620. Thus,according to the exemplary embodiment shown in FIG. 6, the hard maskstructure comprises hard mask layer 624 and cap/etch stop/thin hard masklayer 620.

As described, for example, in conjunction with the description of FIGS.3-5 above, the hard mask structure is configured to have a base,proximate to the material being etched, that has one or more lateraldimensions that are greater than a top of the hard mask structure, suchthat at least one portion of the base extends out laterally asubstantial distance beyond the top. According to one exemplaryembodiment, when viewed in lateral cross section, each portion of thebase extending out laterally beyond the top has a longest lateraldimension that is between about 20 percent and about 40 percent of thelongest lateral dimension of the base. For example, each portion of thebase extending out laterally beyond the top may have a longest lateraldimension that is between about 30 percent and about 40 percent of thelongest lateral dimension of the base.

The top of the hard mask structure is at a greater distance from thematerial being etched than the base. For example, the hard maskstructure may comprise a base, e.g., comprising cap/etch stop/thin hardmask layer 620, and a top, e.g., comprising hard mask layer 624. Thehard mask structure may be a circular, or substantially circular (e.g.,elliptical) structure, wherein the base has a diameter that is greaterthan a diameter of the top of the hard mask structure, distal to thematerial being etched. See, for example, FIGS. 7A-D. Specifically,according to one exemplary embodiment, the hard mask structure iscircular, and has a diameter 628 at its base that is greater thandiameter 630 at its top. Further, each portion of the base extending outlaterally beyond the top has a first slope associated with its uppermostsurface and at least one sidewall of the top has a second slope, thefirst slope being different from the second slope. Specifically, theslope of the top is greater than the slope of the base. For example, theslope of the base and of the top, as shown in step 606 of FIG. 6 b, iszero and infinity (90 degrees), respectively.

In step 608, magnetic layer 614, coupling layer 616 and magnetic layer618 are etched, e.g., to dimensions similar to cap/etch stop/thin hardmask layer 620, using an etch technique that is selective againstetching cap/etch stop/thin hard mask layer 620. In an alternativeembodiment, spacer material 626 would remain until after the magneticlayers are etched. A liftoff would then be performed to remove spacermaterial 626 along with any residual nonvolatile materials that may haveaccumulated on its surface during the etch of the magnetic materials.

EXAMPLES

FIGS. 7A-D are images of hard mask structures graded using the presenttechniques. Specifically, FIGS. 7A and 7C show top-down views and FIGS.7B and 7D show tilted-angle side views of graded hard mask structures.

Each of the hard mask structures shown in FIGS. 7A-D comprise a thickhard mask layer on top of a thin hard mask layer, e.g., hard mask layers702 and 704, respectively. Hard mask layer 702, the thick hard masklayer, comprises TiN. Hard mask layer 704, the thin hard mask layer,comprises TaN. Hard mask layer 704 extends out as a foot beneath hardmask layer 702. The structures were formed in the manner described inFIG. 4, above, e.g., with a chlorine-based RIE to pattern the hard masklayers 702 and 704, and a H₂O₂/NH₄OH wet etch to recess the TiN behindthe foot of TaN.

As described, for example, in conjunction with the description of FIGS.3-6 above, the hard mask structure may comprise a circular, orsubstantially circular (e.g., elliptical), structure, wherein the basehas a diameter that is greater than a diameter of the top of the hardmask structure, distal to the material being etched. For example, asshown in FIGS. 7A-D, hard mask layer 704 has a diameter that is greaterthan hard mask layer 702.

FIGS. 8A-D are also images of hard mask structures graded using thepresent techniques. The images in FIGS. 8A-D reveal the magnetic layers,e.g., of a magnetic device, after etching has been conducted using thegraded hard mask structure. Specifically, FIGS. 8A and 8C show top-downviews and FIGS. 8B and 8D show tilted-angle side views of fully-etchedmagnetic devices with graded hard mask structures. As described inconjunction with the description of FIGS. 7A-D, above, each of the hardmask structures shown in FIGS. 8A-D comprise a thick hard mask layer ontop of a thin hard mask layer, e.g., hard mask layers 802 and 804,respectively. Hard mask layer 802, the thick hard mask layer, comprisesTiN. Hard mask layer 804, the thin hard mask layer, comprises TaN. Hardmask layer 804 extends out as a foot beneath hard mask layer 802.

Reactive ion etching with a CO—NH₃ plasma has been used to etch magneticlayers existing below hard mask layer 804. As shown, hard mask layer 804provided a robust mask against the etch.

In conclusion, the techniques presented herein serve to, at least inpart, prevent unfavorable magnetic device characteristics resulting fromnonvolatile material re-deposited during formation of the device.

Although illustrative embodiments of the present invention have beendescribed herein, it is to be understood that the invention is notlimited to those precise embodiments, and that various other changes andmodifications may be made by one skilled in the art without departingfrom the scope or spirit of the invention.

1. A method of patterning at least one material, the method comprisingthe steps of: forming a hard mask structure on at least one surface ofthe material to be patterned, the hard mask structure being configuredto have a base, proximate to the material, and a top opposite the base,the base having one or more lateral dimensions that are greater than oneor more lateral dimensions of the top of the hard mask structure, suchthat at least one portion of the base extends out laterally asubstantial distance beyond the top, the top of the hard mask structurebeing at a greater vertical distance from the material being etched thanthe base; and etching the material.
 2. The method of claim 1, whereinthe at least one portion of the base extending out laterally beyond thetop has a first slope associated with an uppermost surface thereof andat least one sidewall of the top has a second slope, the first slopebeing less than the second slope.
 3. The method of claim 1, wherein eachat least one portion of the base extending out laterally beyond the top,when viewed in lateral cross section, has a longest lateral dimensionthat is between about 20 percent and about 40 percent of the longestlateral dimension of the base.
 4. The method of claim 1, wherein each atleast one portion of the base extending out laterally beyond the top,when viewed in lateral cross section, has a longest lateral dimensionthat is between about 30 percent and about 40 percent of the longestlateral dimension of the base.
 5. The method of claim 1, wherein thematerial being etched comprises at least one nonvolatile material. 6.The method of claim 1, further comprising the step of reducing one ormore lateral dimensions of the top of the hard mask structure.
 7. Themethod of claim 1, further comprising the step of reducing one or morelateral dimensions of the top of the hard mask structure by etching. 8.The method of claim 1, further comprising the step of reducing one ormore lateral dimensions of the top of the hard mask structure by use ofone or more of selective wet etching and selective reactive ion etchingtechniques.
 9. The method of claim 1, further comprising the step ofutilizing one or more disposable spacer materials to achieve the atleast one portion of the base extending out laterally a substantialdistance beyond the top.
 10. The method of claim 8, wherein the one ormore disposable spacer materials comprise one or more of organicpolymers, dielectric materials, and metals.
 11. The method of claim 1,wherein etching of the material comprises use of one or more of reactiveion etching and ion beam etching.
 12. The method of claim 1, wherein thematerial comprises one or more layers of a magnetic device.
 13. A methodof forming a device comprising at least one nonvolatile material, themethod comprising the steps of: forming a hard mask structure on atleast one surface of the material to be patterned, the hard maskstructure being configured to have a base, proximate to the material,and a top opposite the base, the base having one or more lateraldimensions that are greater than one or more lateral dimensions of thetop of the hard mask structure such that at least one portion of thebase extends out laterally a substantial distance beyond the top, thetop of the hard mask structure being at a greater vertical distance fromthe material being etched than the base; and etching the material. 14.The method of claim 13, wherein the hard mask structure comprises anelectrically conductive material at least a portion of which serves as acontact in the device.
 15. The method of claim 13, wherein the devicecomprises a magnetic device.
 16. The method of claim 15, wherein thenonvolatile material comprises one or more magnetic layers.
 17. Themethod of claim 13, wherein the device comprises one or more offerroelectric random access memory devices, phase change memory devices,and complementary metal oxide semiconductor devices.
 18. A hard maskstructure comprising: at least one hard mask component configured toimpart on the hard mask structure a base having one or more lateraldimensions that are greater than one or more lateral dimensions of thetop of the hard mask structure, such that at least one portion of thebase extends out laterally a substantial distance beyond the top. 19.The hard mask structure of claim 18, wherein each at least one portionof the base extending out laterally beyond the top, when viewed inlateral cross section, has a longest lateral dimension that is betweenabout 20 percent and about 40 percent of the longest lateral dimensionof the base.
 20. The hard mask structure of claim 18, wherein each atleast one portion of the base extending out laterally beyond the top,when viewed in lateral cross section, has a longest lateral dimensionthat is between about 30 percent and about 40 percent of the longestlateral dimension of the base.
 21. The hard mask structure of claim 18,wherein the at least one hard mask component has a taperedconfiguration.
 22. The hard mask structure of claim 18, wherein the atleast one hard mask component comprises a pair of hard mask layers. 23.The hard mask structure of claim 18, wherein the at least one hard maskcomponent comprises a pair of hard mask layers, one of the pair of hardmask layers having a thickness greater than an other of the pair of hardmask layers.
 24. The hard mask structure of claim 18, wherein the atleast one hard mask component comprises a pair of hard mask layers, oneof the pair of hard mask layers comprising TiN and another of the pairof hard mask layers comprising TaN.
 25. The hard mask structure of claim18, comprising a single hard mask layer.